Belief in the practice of witchcraft is widespread in Malawi. Photograph: Martin Godwin for the Guardian

Dozens of people in Malawi, most of them elderly women, have been jailed for up to six years with hard labour for practising witchcraft.

Campaigners say they will call on the president, Bingu wa Mutharika, to release the 86 prisoners since witchcraft is not a crime under Malawian law.

Most of the group are elderly women accused by children of teaching them witchcraft. Belief in witches is widespread in Malawi.

George Thindwa, spokesman for the Association of Secular Humanism, called for the women to be freed immediately because they had committed no crime.

"We are intervening in this matter because we are concerned we still have prisons in Malawi [with] people being accused of being witches," he told the BBC's Network Africa programme. "The courts were wrong 100%, [and] the police, to actually accommodate cases."

Thindwa said the women were vulnerable and their convictions had taken place with undue haste. "The problem is that our police and our courts, most of them are witchcraft believers and this belief is very strong here in Malawi."

Last year the government bowed to public pressure by setting up a committee to investigate criminalising witchcraft.

Malawi's public prosecutions office told the BBC that 11 cases were brought under the witchcraft act in the last month. This led to the conviction of 61 elderly women, seven elderly men and 18 younger relatives of the other accused. They received prison sentences of between four and six years.

Justice minister George Chaponda has claimed that a person can only be found guilty of practising witchcraft if they confess to being a witch. But the BBC reported that the records showed all the suspects had pleaded not guilty.

Thindwa said: "I'm happy the minister has invited anybody with evidence to come forward. We have a complete dossier of the cases we are disputing. We'll take the dossier to his office immediately."

There has been an increase in allegations of witchcraft in parts of central and west Africa, with penalties including ostracisation, violence and execution. A recent report by Unicef found tens of thousands of children, some as young as four, were being accused of being witches.